Neopentyl glycol,2,2-dimethyl-1,3-dihydroxypropane, is a diol of substantial value in the commercial chemical arts useful in a variety of formulations in lubricants and adhesives, coatings, plastics and fibers. As a key component in many chemical products, the cost and purity of neopentyl glycol is a key determinant in the eventual competitiveness of the products produced therefrom. Accordingly, those skilled in the chemical arts applicable to neopentyl glycol regularly search for improvements in the process for the production of the diol to advance the technology and usefulness of the products produced.
Due to the usefulness of neopentyl glycol and to certain complexities intrinsic to its manufacture the chemical has been the subject of a substantial number of investigations directed toward improved manufacturing processes and consequently, the prior art is crowded. Although the basic chemistry of the synthesis of neopentyl glycol appear quite straight forward, when incorporated in a process directed toward maximum economic yield challenging problems are presented.
The basic reaction for the most common processes for the manufacture of neopentyl glycol is represented by the equation: ##STR1## wherein isobutyraldehyde and formaldehyde are allowed to react through aldol condensation in the presence of a basic catalyst to yield 2,2-dimethyl-3-hydroxypropanal (hydroxypivaldehyde). This intermediate is then catalytically hydrogenated to produce neopentyl glycol.
The synthesis of neopentyl glycol has been the subject of reviews in the chemical literature, in particular the review by Cornils and Feischtinger in Chem Zeitung, 100 (12) 504 (1976). Catalysts employed to affect the aldol condensation have included alkali hydroxides, alkaline earth metal hydroxides, alkali carbonates, tertiary amines and basic ion exchangers. Of these aldol catalysts, the use of alkali hydroxides, alkali carbonates and tertiary amines has received the most study in the literature. The object of the search for improved catalysts is the reduction or elimination of side reactions that compete with the desired aldol condensation and produce impurities in the reaction mixture requiring additional process steps for their removal. The competing side reactions are due to Cannizzaro and Tischenko reactions where, in the former, two moles of aldehyde react to produce a mole of alcohol and a mole of carboxylic acid and, in the later, two moles of aldehyde react to form the corresponding ester. To the extent these competing reactions occur, the yield and purity of the desired product are not satisfactory and processes based on this overall reaction scheme, such as U.S. Pat. No. 2,400,724, are not generally considered economically viable.
To moderate these competing reactions which occur particularly in the presence of alkali hydroxides sodium carbonate has been used as described in U.S. Pat. No. 2,811,562. Although improvements have been found for aldol catalysts that reduce side reactions, these catalysts may complicate the subsequent hydrogenation step by requiring separation of the intermediate hydroxypivaldehyde before hydrogenation or may dictate the selection of a hydrogenation catalyst of intermediate activity, thereby reducing the final yield of neopentyl glycol. Efforts to resolve this problem have been described in UK Pat. No. 1,017,618 as well as UK Pat. No. 1,048,530 wherein 2,2-dimethyl-3-hydroxypropanal is hydrogenated simultaneously with isobutyraldehyde in the presence of a copper/chromium oxide catalyst. This process is reported to lead to the formation of appreciable quantities of a large number of by-products.
The use of tertiary amines as the aldol catalyst for the condensation of formaldehyde and isobutyraldehyde is also described in Dutch Pat. No. 6,405,068 and in U.S. Pat. No. 3,808,280. U.S. Pat. No. 3,808,280 claims the use of triethylamine as the aldol catalyst and the subsequent use of catalysts containing at least one of cobalt, copper, manganese and nickel to convert the hydroxypivaldehyde to neopentyl glycol and teaches a mixture of these metals is the preferred catalyst. The use of triethylamine and a copper-containing hydrogenation catalyst is claimed; however, it is indicated that copper chromite would be a poor hydrogenation catalyst because of decomposition of hydroxypivaldehyde to produce excessive by-products and lowered yield of neopentyl glycol. Further, the hydrogenation step as described is U.S. Pat. No. 3,808,280 is conducted between 1,500 and 4,000 psig.